In my copending application Ser. No. 456,416, filed Jan. 7, 1983, now U.S. Pat. No. 4,468,239, there is disclosed a twin tower assembly for decontaminating compressed gas, the towers of which each contain a desiccant and under timer control of individual solenoid valves alternately receive contaminated gas and individually cycle between decontaminating and regenerating cycles. Whether the source is a compressor or a reservoir, the compressed gas usually contains as principal contaminants water and oil, the latter in the form of a mist or aerosol dispersion and larger particles entrained in the gas. The desiccant in the towers preferably is a molecular sieve suitable for adsorbing moisture and, if exposed to oil, such a sieve is rapidly contaminated by blocking of its pores. It therefore is vital that oil entrained in the gas undergoing decontamination be removed before the gas reaches the desiccant. In the above application this is accomplished by containing in each tower upstream of the desiccant a hollow cylinder or tube of knitted wire mesh, on passing of the gas through the side wall of which from outside to inside, entrained oil in the form of mist and fine particles is coalesced and a combination of gravity drainage and, during a regenerating cycle, a reverse flow of clean purge gas, are depended on for purging the coalescer of oil trapped therein. The present application is primarily directed to an improvement on the assembly of the above application in the removal of entrained oil in advance of the desiccant.